The present invention relates to a centrifugal separator comprising a rotatable centrifugal rotor, which delimits a separation chamber having peripheral outlets for a separated substance, a stationary inlet device for introducing into the centrifugal rotor a liquid mixture that contains said substance and that is to be treated in the separation chamber, an outlet device which is rotatable with the centrifugal rotor and adapted to open and close said peripheral outlets intermittently during rotation of the centrifugal rotor for discharging said separated substance from the separation chamber and an actuation device arranged outside the centrifugal rotor and adapted to actuate said outlet device so that it maintains the peripheral outlets open to an extent and/or during a time such that a predetermined amount of said separated substance leaves the centrifugal rotor, said extent and/or time being variable. Particularly, the invention concerns a control device for keeping constant the said amount of separated substance leaving the separation chamber each time said peripheral outlets are opened and closed.
It is long known to use in connection with a centrifugal separator of the above defined kind sensing means by which it may be determined exactly when a certain amount of said separated substance has been accumulated in the centrifugal rotor separation chamber and, then, to open and close automatically said peripheral outlets. However, it has proven difficult during a separating operation to accomplish discharge of like amounts of such separated substance through the peripheral outlets each time these are opened and closed.
The reason for this difficulty seems to be that a centrifugal rotor of the kind here in question has a poor ability of maintaining the peripheral outlets open to the same extent and/or during the same time at each discharge operation and, therefore, to discharge alike amounts of substance at the various opening times. Thus, if a relatively small amount of substance is discharged, the substance if it is constituted by solid particles is given time to get a too high concentration of such particles before it is discharged through the peripheral outlets. This can lead to the effect that part of the separated substance is given time to fasten onto the inside of the walls of the centrifugal rotor before the peripheral outlets are opened. If, on the other hand, a relatively large amount of substance is discharged, the substance gets a too low concentration of particles, i.e. the discharged substance contains an undesired amount of the liquid from which the particles should be separated. This can lead to undesired losses, since it is often the liquid that is the valuable part of the mixture being supplied to the centrifugal rotor.
Even in separation cases where prior to a sludge discharge operation a valuable separated liquid is displaced radially inwardly in the separation chamber, by supply to the separation chamber of a certain amount of a less valuable liquid having a higher density than the valuable liquid, it may be of value that a well controlled amount of separated substance (particles and/or liquid) is discharged through the peripheral outlets each time these are opened and closed. Thus, it becomes possible to optimize the amount of added less valuable liquid before every time the peripheral outlets are to be opened. The supply of unnecessarily much liquid of this kind takes an undesired time into account during which the separating operation is interrupted.
For resolving the above discussed problem to discharge a predetermined amount of separated substance through the peripheral outlets, each time these are opened and closed, design improvements have been constantly made of said outlet device of the rotor and of the actuation device situated outside the centrifugal rotor for actuation of the outlet device. However, this has not completely resolved the problem. Previously known devices for accomplishing a desired discharge of separated substance from a centrifugal rotor of the kind here in question is described for instance in U.S. Pat. No. 4,510,052 and WO 97/27 945.
The object of the present invention is to provide a control device by means of which the amount of separated substance leaving the separation chamber in a centrifugal rotor of the kind initially defined can be kept substantially unchanged every time the outlet device opens and closes the peripheral outlets.
This object can be fulfilled according to the invention by means of a regulation device that is characterized by a sensing device for sensing of a parameter representative for the amount of mixture supplied per unit of time into the centrifugal rotor through said inlet device, and a control device which is connected both to the sensing device and to said actuation device, the control device being adapted to receive from the sensing device a signal reflecting the amount of mixture, which per unit of time is supplied into the centrifugal rotor, and in response to said signal to control the actuation devicexe2x80x94in accordance with a predetermined relation between the amount of mixture supplied per unit of time into the centrifugal rotor through the inlet device and the extent and/or the time that the peripheral outlets are to be maintained open by means of the outlet devicexe2x80x94such that said predetermined amount of the separated substance leaves the centrifugal rotor. The said relation in certain cases may be calculated but in other cases has to be determined empirically.
Conventionally, in connection with a centrifugal separator of the kind here in question, the said outlet device is adapted to actuate one or more valves or slides of the centrifugal rotor by means of a fluidxe2x80x94liquid or pressurized airxe2x80x94supplied to the centrifugal rotor by means of said actuation device outside the centrifugal rotor. An outlet device of this kind may be used with advantage even in connection with the present invention. However, within the scope of the invention, even electrically, magnetically, thermally or otherwise actuatable outlet devices may be useful.
Upon use of an outlet device that is actuatable by means of a supplied fluid this outlet device may be of various kinds. Thus, the outlet device may be adapted to open said peripheral outlets and keep these open to a varying extent or during a varying time depending upon the pressure by which said fluid is supplied by means of the actuation device. Alternatively, the outlet device may be adapted to operate in dependence of the amount of fluid or the amount of fluid per unit of time delivered by the actuation device.
Even the actuation device may be of most varying kinds. In a case where said fluid is a liquid and such liquid is to be supplied to the outlet device at a variable but predetermined pressure, the actuation device may include a container for the liquid and a movable body, e.g. a piston, within the container for displacement of the liquid out of the container. Further, the actuation device may include a container for pressurized air, possibly formed by part of the container for liquid.
The above said sensing device may be constituted by a conventional mass or volume flow meter or by any suitable kind of equipment which directly or indirectly is able to sense the magnitude of a liquid flow through the inlet device of the centrifugal separator, e.g. a pressure meter. The sensing device should be adapted to emit a signal of any suitable kind that is representative for the magnitude of the sensed liquid flow. The signal may have the form of an electric current or voltage, the magnitude of which is dependent on the magnitude of the sensed liquid flow.
The control device which shall receive the signal generated by the sensing device should be adapted in one way or another to control the aforementioned actuation device. The way in which such a control is accomplished is, of course, dependent on the type of actuation device having been chosen.
In a very simple case the actuation device may be adapted to supply through a conduit a so called operating liquid having a certain pressure. Then, a closing valve may be present in said conduit, which valve may be opened and be kept open during a certain controllable period of time. By means of an actuation device of this kind a desired amount of liquid may be supplied to the centrifugal rotor during a desired period of time. Whereas the opening movement of said valve may be initiated by an equipment adapted to determine the point of time when the peripheral outlets of the centrifugal rotor are to be opened, the time for keeping the valve open may be controlled by said control device in dependence of the signal from the sensing device.
In another case, which is described in detail below with reference to the accompanying drawings, the actuation device may be adapted to deliver a liquid flow by means of pressurized air, the pressure of which is variable. Thus, the magnitude of said air pressure may be controlled by means of the control device in dependence of the signal from the sensing device, the actuation device further being adapted to supply only a certain amount of so called operating liquid during a time that is dependent on the chosen air pressure.
Other possibilities are described in the aforementioned patent specifications U.S. Pat. No. 4,510,052 and WO 97/27 945.